The present invention relates to an infrared photodetector, more specifically, a quantum dot infrared photodetector using quantum dots in the infrared sensor.
As one of the infrared photodetectors, the quantum dot infrared photodetector using quantum dots in the infrared sensor is proposed. In comparison with the quantum well infrared photodetector using quantum wells in the infrared sensor, the quantum dot infrared photodetector has the merits of high detection sensitivity to the infrared radiation incident perpendicularly to the device surface, low probability of photo-excited carriers being recaptured and accordingly high photocurrent gain, and other merits. Thus, the quantum dot infrared photodetector is expected as an infrared photodetector providing large photocurrent, i.e., high sensitivity.
In the photodetector of large photocurrent, signal current can be detected even at relatively high operation temperatures without being buried in dark current (which increases exponentially with respect to the operation temperature) which is a noise source, and it is less necessary to cool the photodetector so as to suppress the dark current. Accordingly, the cooler can be simplified, and the infrared photodetector can be accordingly smaller-sized and can have lower costs.
The quantum dot infrared photodetector is disclosed in, e.g., Japanese published unexamined patent, application No. 10-256588 (hereinafter called Reference 1), and V. Ryzhii, “The theory of quantum dot infrared photodiodes”, Semicond. Sci. Technol., Vol. 11, 1996, p. 759 (hereinafter called Reference 2).
As the proposed quantum dot infrared photodetector is known a quantum dot infrared photodetector using InAs or InGaAs, etc. as the quantum dots, and GaAs as the intermediate layer burying the quantum dots and the electrode layers. However, when such quantum dot infrared photodetector is actually manufactured, the quantum dot infrared photodetector does not provide the sensitivity as expected.
The quantum confinement in the quantum dots, which determines the operation of the infrared photodetector is determined by the potential barrier height between the InAs or InGaAs, etc., which is the material of the quantum dots, and the GaAs intermediate layer surrounding the quantum dots. The sensitivity which is lower than expected will be due to one cause that the potential barrier height for the quantum confinement is insufficient. To improve the sensitivity of the quantum dot infrared photodetector, it will be effective to use a material whose band width is larger than the GaAs intermediate layer, e.g., an AlGaAs intermediate layer.
However, it has been found that although such structure surely generally improves the sensitivity, the sensitivity improvement in the low current region, where the signal noise characteristics are good, is relatively lowered.
FIG. 10 is a graph schematically showing the relationships between the sensitivity and the dark current with the operation voltage of the quantum dot infrared photodetector varied. In the graph, the dotted line indicates the case that the intermediate layer is formed of GaAs, and the solid line indicates the case that the intermediate layer is formed of AlGaAs.
In the case that the intermediate layer is formed of AlGaAs, whose band width is larger than that of GaAs, the characteristics having the same inclination as the case that the GaAs intermediate layer (indicated by the broken line in the graph) is estimated. In the actually manufactured devices, however, as indicated by the solid line, the sensitivity relatively lowers in the low current region. The infrared photodetector uses, in many cases, such low current region so as to obtain good signal noise characteristics, and the relative reduction of the sensitivity in this region is a serious problem.